This invention relates to chemical-mechanical planarization of semiconductor wafers, and more particularly to fluid flow regulating systems used in such machines.
As the level of integration increases on semiconductor wafers, surface irregularities on the wafer have become a serious problem. For example, metallization layers used to form interconnects between the various devices on the wafer may lead to substantial surface irregularities that interfere with the performance of subsequent photolithographic steps on the wafer. In order to flatten these surface irregularities, numerous materials or methods have been developed, such as SOG (Spin on Glass), and reflow. However, since these methods cannot globally planarize the wafer surface and may not sufficiently remove wafer surface irregularities, they have largely given way to the use of polishing techniques to planarize the surface of semiconductor wafers.
In one commonly used technique, known as chemical-mechanical planarization, the semiconductor wafer is mounted in a wafer carrier, and a polishing pad is held on a platen that can be rotated. The exposed surface of the wafer is then pressed against the polishing pad with a prescribed down force, and the polishing pad and/or the wafer are then independently rotated while the wafer carrier is translated across the pad surface. The process is continued until the desired degree of surface uniformity on the wafer is attained. In this technique, the abrasive mechanism is generally provided by a planarization fluid that contains abrasive particles in suspension with a combination of chemical etchants that are formulated to etch and dissolve certain materials that comprise the wafer. Alternatively, the planarization fluid may contain only the chemical etchants, with the abrasive elements embedded in a xe2x80x9cfixed abrasivexe2x80x9d pad.
The planarization fluids used in chemical-mechanical planarization are most commonly supplied to wafer manufacturers in a commercially prepackaged form, which may be comprised of two or more parts that are combined prior to planarizing a production run of wafers. Once the components are mixed, the planarization fluid is distributed to wafer planarization machines by a planarization fluid distribution system. Numerous disadvantages are present in planarization fluid distribution systems which are explained more fully with reference to the structure and operation of a typical prior art planarization fluid distribution system 10 which is shown in FIG. 1.
With reference now to FIG. 1, carefully measured volumes of planarization fluid components 130 and 132 are combined in a mixing tank 138 to form a planarization fluid 14. The mixing tank 138 has a mechanical agitator 136 that is driven by an electric motor 134 to mix the components and to keep the abrasive component of planarization fluid 14 in suspension. After the planarization fluid 14 has been sufficiently mixed, the planarization fluid 14 is transferred to a storage tank 12 through line 120. The storage tank 12 has an outlet pipe 18 for transferring planarization fluid 14 from the tank 12 to a planarization fluid distribution loop 140. A peristaltic pump 124 that is driven by a motor 122 pumps planarization fluid around the distribution loop 140. Planarization fluid distribution branches 160a-160d allow planarization fluid 14 to be distributed to planarizing machines 126a-126d, and the amount of planarization fluid 14 distributed to the machines 126a-126d may be controlled by manually actuated valves 150a-150d. Although only four planarization machines are shown for clarity of presentation, a larger number of machines may be present in a typical system. By maintaining constant fluid motion in the distribution loop 140, abrasive settling in the distribution loop 140 is avoided. Moreover, the constant pumping of planarization fluid 14 from storage tank 12 to the distribution loop 140, together with the return of the unused portion of the planarization fluid 14 to the storage tank 12 through return pipe 16 may keep the abrasive components of planarization fluid 14 sufficiently agitated.
One disadvantage of the prior art fluid distribution system 10 is that it will not permit planarization fluids to be mixed from constituent components close to the machine. The mixing and use of planarization fluid on an as-needed basis is advantageous because the chemical etchants present in the fluid are subject to chemical degradation, and should be used relatively soon after mixing occurs. The combination of fluid components at the machine will generally permit smaller volumes to be mixed which may be more completely consumed in the wafer planarizing process, thus minimizing the waste of planarization fluid.
Another disadvantage of the prior art distribution system 10 is that it cannot accurately regulate the amount of planarization fluid delivered to each machine. Referring again to FIG. 1, a peristaltic pump 124 is used to deliver the planarization fluid 14 to the machines 126a-126d. Since the peristaltic pump 124 is sensitive to changes in the fluid level in the tank 12, the amount of fluid delivered to machines 126a-126d will vary as the planarization fluid 14 is used. Consequently, the delivery of planarization fluid to machines 126a-126d in uniform, precisely regulated amounts cannot be readily accomplished in the prior art system 10.
Still other problems are inherent in the prior art planarization fluid distribution system 10. For example, the prior art planarization fluid distribution system 10 requires a minimum volume of planarization fluid 14 in order to operate, and depending on the size of the system, this volume may be considerable. With reference again to FIG. 1, it is seen that the planarization fluid distribution system 10 requires that the distribution loop 140 be filled with planarization fluid 14 during operation, and that the storage tank 12 contain a sufficient volume of planarization fluid to permit pumping from the storage tank 12. Consequently, when all wafer planarization processing is completed, a significant volume of unused planarization fluid is retained within the system 10. Since the unused planarization fluid loses its effectiveness over time, it cannot be retained for use in planarizing subsequent wafer production runs and is generally discarded. This waste contributes to the overall cost to produce the wafer since commercially available planarization fluid formulations are relatively costly. Still other costs are incurred in discarding the excess planarization fluid, because it must be disposed of as toxic waste.
Still other disadvantages are associated with the prior art planarization fluid distribution system 10. For example, after the removal and disposal of the excess planarization fluid, the entire distribution system is flushed with deionized water to remove the remaining fluid. However, flushing the distribution system presents still other waste disposal problems since the water used to flush the system generally contains significant concentrations of chemical constituents, as well as abrasives. It must therefore be processed to remove these materials before the water can be discharged into a municipal wastewater disposal system. An additional problem associated with flushing the system is that there is usually no way to remove the de-ionized water that is retained in the distribution system after it is flushed and drained. If the distribution system has a significant volume, considerable amounts of water will remain in the system after flushing. Consequently, the water retained by the system will dilute the fresh planarization fluid mixture when it is transported through the system. This diluted planarization fluid may cause inconsistent planarization results in subsequent wafer production runs.
Finally, abrasive settling problems are not effectively addressed by the prior art planarization fluid distribution system 10. Abrasive settling, in particular, is a significant problem in wafer planarization because abrasive-rich mixtures generally form in regions near the bottom of storage vessels, mixing tanks and distribution lines. Once formed, these mixtures may lead to uneven planarization of the wafer, or cause the wafer to be planarized beyond the desired endpoint. Moreover, if the abrasive settling is not controlled, large agglomerations of abrasive particles may ultimately form in the planarization fluid that may lead to surface scratching of the wafer. Although the prior art distribution system 10 uses a distribution loop 140 to inhibit abrasive settling, abrasive particles may still settle in locations that are not subject to recirculation. For example, since wafer planarization generally occurs on a periodic basis, the machines must be stopped in order to remove planarized wafers from the wafer holders and to load unprocessed wafers into the wafer holders. During this period, the flow of planarization fluid 14 from the distribution loop 140 to the machines 126a-126d is stopped by closing valves 150a-150d, which allows the planarization fluid 14 to remain stationary within the distribution branches 160a-160d, thus allowing the abrasives to settle and agglomerate. Reestablishing movement of the planarization fluid in the distribution lines will not, in general, significantly break up these agglomerations once they have formed.
Many of the shortcomings inherent in prior art planarization fluid distribution systems could be eliminated if the fluid could be supplied to the planarization machines individually from a point-of-use planarization fluid distribution system. As used herein, the term xe2x80x9cpoint-of-usexe2x80x9d refers to a fluid distribution system that is located in proximity to the planarization machine that supplies planarization fluid to an individual planarization machine.
Since the point-of-use system is located in proximity to the machine, the need for long distribution lines and recirculation loops is eliminated. Further, since a point-of-use system supplies planarization fluid to individual planarization machines, the internal volume of the system can be small. Consequently, many of the large volume components associated with the prior art planarization fluid distribution systems, such as recirculating loops, large mixing containers and storage tanks are eliminated. As discussed above, the large volume components comprising the prior art distribution system are generally recognized as significant contributors to planarization fluid waste and system cleaning difficulties.
A point-of-use system capable of precise flow regulation will also eliminate planarization fluid flow regulation problems that stem from the input pressure sensitivity inherent in peristaltic pumps, thereby permitting a more efficient utilization of planarization fluid. Precise flow regulation will additionally permit the components of a multi-component planarization fluid to be combined just prior to depositing the mixture on the planarization pad so that fluid is supplied on an as-needed basis, which greatly reduces waste.
Other advantages of the invention will become apparent based upon the description of the invention provided below when read with reference to the drawing figures.
The present invention is directed to an apparatus and method for planarization fluid flow regulation that allows point-of-use distribution of a planarization fluid to a semiconductor wafer planarization machine. In one aspect, the regulating apparatus includes a planarization fluid storage tank with an acoustic fluid level sensor to detect the fluid level within the storage tank. The storage tank is connected to a planarization fluid delivery line that delivers planarization fluid to the storage tank through a flow control valve and delivers a regulated flow of planarization fluid to a planarization machine through a flow measurement device. A gas supply system is connected to the storage tank to provide system pressurization. Regulation of the planarization fluid flow from the regulating apparatus is achieved by a control system in which the flow measurement device and the acoustic fluid level sensing capability comprise feedback elements in a closed feedback system to independently control the pressure in the storage tank and the amount of fluid admitted by the control valve. In an alternate aspect, the fluid level sensor is comprised of an array of capacitive proximity sensors located outside the wall of the storage tank. In another alternate aspect, the fluid level sensor is replaced by a buoyant float that is partially buoyant in the planarization fluid that is adapted to seat in the upper or lower ends of the storage tank when the storage tank is full or empty. Indication of the full and empty tank conditions are obtained from a differential pressure sensor which is suitably located to sense differences in the storage tank pressure when the float is seated in either the upper or lower end of the storage tank. In still another aspect, two or more regulators may be joined in a parallel flow arrangement in order to achieve precise point-of-use mixing and flow rate control of multi-component planarization fluids.